The present invention relates to supported ultrathin copper foil, to a method of producing copper clad laminates therefrom and to the laminates so produced. The terms "ultrathin" and "non-self-supporting" are used herein to describe copper layers having thicknesses no greater than about 12 microns. Foils of such thinness are subject to damage under the force of their own weight if unsupported.
Printed circuit elements are commonly manufactured from copper clad laminates by means of an etching process. The copper clad laminates used in the manufacture of printed circuits are commonly forced from copper foil and a substrate insulating material by application of heat and pressure in a laminating press. In the standard etching process, the copper clad surface is coated with a photoresist material and then covered with a mask defining the desired circuitry. The photoresist is then exposed in light causing that portion representing the desired circuitry lines to develop and harden. The undeveloped photoresist is washed away and the masked surface is then treated with an etching solution to remove the unwanted, exposed portions of the copper.
A problem commonly encountered in such processes is termed "under-cutting" wherein the etching solution during removal of the unwanted copper attacks the mask-protected copper lines from the side, below the masking. This problem is particularly acute where it is desired to produce a circuit having very fine lines.
One solution to this problem is to employ a very thin layer of copper cladding. As the thickness of the copper cladding is decreased, further advantages accrue, i.e. the necessary etching time is decreased and the problems associated with the disposal of the spent copper-containing etching solution are diminished.
Thus, there exists a demand for a process suitable for the commercial production of ultrathin foils in quantity. However, in the manufacture of ultrathin foils, unique problems, not associated with the production of thicker foils, are encountered. These problems may be categorized as (1) difficulties associated with the nature and limitations of the plating surface and (2) difficulties encountered in handling and working with a non-self-supporting ultrathin foil product.
Several solutions directed to the latter category of difficulties have previously been proposed. U.S. Pat. No. 2,105,440 issued in 1938 to Miller recognized that the thinness and lack of mechanical strength of such ultrathin foils precluded conventional methods of foil manufacture wherein the foil is mechanically stripped from a cathode drum or plate. Miller proposed to increase the mechanical strength of ultrathin foils by affixing a fibrous or paper backing to the exposed surface of the foil prior to removal of the foil from a rotating cathode. The method of Miller closely resembles that disclosed in an earlier patent, U.S. Pat. No. 454,381 issued to Reinfeld in 1891. However, the concept of gluing a paper to an electrode deposit on the face of a moving drum presents other problems. For example, Miller uses a quick-setting resinous adhesive because of the short time available for the adhesive to set before removal from the cathode which must move at a commercially feasible speed. The difficulty of removal of such resinous adhesives would present a significant problem if a foil such as that produced by Miller were to be used in the production of printed circuit elements. The thin copper layer of Miller, if laminated on its free surface to a second resinous substrate, would show little preference for adherence to one or the other substrate. Even if one were to devise a method for selectively removing the paper and first resin without tearing the thin copper layer, the surface of the thin copper would still require cleaning to remove traces of the first resin.
The process described in the copending application of Adam M. Wolski, U.S. Ser. No. 354,196 filed Apr. 25, 1973 and entitled "Thin Foil", adopts an entirely different approach to the problem addressed by Miller and Reinfeld. The process of such copending application circumvents the problems of separating the thin foil from the cathode by using a disposable cathode or temporary carrier and leaving the thin foil attached to the carrier through a laminating step wherein the thin foil is laminated to and thus supported by a resinous substrate prior to removal of the original carrier. The foil of this earlier invention comprised an ultrathin layer supported by aluminum and separated therefrom by an intermediate anodized layer of aluminum oxide. Such a composite foil offers a highly desirable advantage in laminating applications in that aluminum and aluminum oxide separate cleanly from the laminate without leaving a residue on the copper surface. Moreover, the thick anodized release layer allows relatively easy separation, thus minimizing the danger of tearing the ultrathin foil.
The process of such prior application successfully overcomes those problems associated with the thinness and lack of mechanical strength of the thin foil. However, the plating of copper on aluminum is limited to the use of copper plating solutions which will not attack or adversely affect the aluminum or aluminum oxide plating surface. The temporary aluminum carrier is relatively immune from attack in copper fluoborate and copper pyrophosphate plating baths, but is not suited for use in acid/copper plating baths or in cyanide plating because the acidic and caustic electrolytes will dissolve the protective aluminum oxide. Unfortunately for the commercial prospects of this process, acidic copper plating baths are the most widely used and commercially preferred for several reasons. An acid/copper bath allows the use of higher cathode current densities than are feasible in connection with cyanide and pyrophosphate plating baths. Fluoborate baths require the use of soluble anodes of refined copper and involve a more sensitive and complex plating process. The plating of copper from an acid bath gives a crystalline structure that is preferred for use in printed circuit laminates because of the higher adhesiveness to a resinous substrate which it provides. Another important advantage is that the acid copper plating bath provides a convenient means for recovery and reuse of the copper scrap. In such a system the scrap copper is merely added to an acid bath wherein it is dissolved with the help of aeration to form copper sulphate. In contrast, in other systems the copper must be first converted to another salt form by an unrelated process before it can be reused in the copper plating bath. Although waste disposal represents a problem with acidic copper plating baths, these disposal problems are not regarded as severe as those associated with other commonly used copper plating systems.
Thus, a need exists in the art for a composite foil that will offer the advantages of aluminum/copper foil composites, i.e. clean separation from the thin copper clad laminate, and which can be manufactured using an acidic copper plating bath.
It is well known that a copper sulfate/sulfuric acid electrolyte cannot be used to plate copper on surfaces of metals which would displace copper from solution. Thus, aluminum and zinc are not sufficiently immune from attack to serve as suitable plating surfaces in an acidic electrolyte without special treatment. Metals such as chromium, steel, nickel and lead, commonly employed as plating surfaces for copper in acidic systems, are either too expensive for use as disposable carriers in the manner contemplated by said earlier application U.S. Ser. No. 354,196 or are otherwise impracticable.